In the hydrostatic extrusion of a billet of a metallic material, the billet is surrounded by a pressure medium which exerts the required pressure on the billet such that the material of the billet is pressed out through the die of the press to form a tube or rod-like product. The presses which are normally used in this connection comprise a pressure chamber for the pressure medium, composed of a high-pressure cylinder, a die arranged in one opening of the high-pressure cylinder for forming the material of the billet, and a punch insertable into the other opening in the high-pressure cylinder for generation of the necessary pressure in the pressure medium. The high-pressure cylinder is subjected internally to very great and highly varying stresses, which leads to fatigue of the portion of the high-pressure cylinder located nearest to the pressure chamber. The high-pressure cylinder is therefore normally constructed with an internal liner in the form of a tube which is exchangeable and can therefore be replaced with a new liner when consumed through fatigue breakdown, or when fatigue breakdown may be expected. To make the liner as resistant as possible to stresses, it is normally manufactured from a high-tensile steel, and it is usually highly prestressed utilizing the surrounding parts of the high-pressure cylinder. The pressure medium which is most used is castor oil, which, besides a good lubricating effect, has the advantage that the viscosity is changed to a relatively small extent with the pressure. For supplying pressure medium to the pressure chamber, the chamber is provided with with an inlet for pressure medium.